Bruker AVANCE, User Manual

Page: 15 / 110

Safety
H157655_1_008 15
2.3.3 Oxygen Level Sensors
Oxygen (O
2
) monitors, or level sensors, are required in the magnet room to detect low levels
of O
2
due to cryogenic gases. At a minimum the following sensors must be provided:
• One oxygen level sensor must be above the magnet, to detect low oxygen levels caused
by high helium gas levels.
• One oxygen level sensor approx. 30 cm off the floor of the magnet room.
• One additional oxygen level sensor approx. 30 cm off the bottom of the pit, in case the
magnet is located inside a pit.
These monitors and sensors generally must be located outside the 0.5 mT (5 G) line. Check
with original equipment manufacturer for information on the effects of magnet fields on these
devices.
Please refer to Emergency Ventilation During Installation and Quenches [
}
68] for more
information on ventilation and exhaust solutions.
2.4 Safe Handling of Cryogenic Substances
Superconducting NMR magnets use liquid helium (all magnets) and nitrogen (only non-Aeon
magnets) as cooling agents, keeping the magnet core at a very low temperature. The safe
handling of cryogenic liquids requires some knowledge of the physical properties of these
liquids, common sense, and sufficient understanding to predict the reactions of such liquids
under certain physical conditions.
Cryogenic liquids, even when kept in insulated storage vessels (dewars), remain at a
constant temperature by their respective boiling temperature. As a result, a fraction of the
liquid constantly evaporates into the gas phase, leading to a pressure build-up inside the
storage dewar. A very important characteristic of cryogens is their enormous increase in
volume during the conversion from liquid to gaseous phase. This conversion follows a raise in
gas temperature starting at the boiling temperatures of the cryogenic liquids and going up
towards room temperature.
The gases are nontoxic and completely harmless as long as adequate ventilation is provided
to avoid suffocation. During normal operation only a small hourly rate of cryogen is
evaporated, but during a quench, an extremely large quantity of helium gas is produced
within a short time.
Cryogenic liquids must be handled and stored in well ventilated areas . Containers for
cryogenic liquids must be constructed with non-magnetic materials and should be specifically
designed for use with particular cryogens. Be sure to read and follow any specific instructions
provided by the container manufacturer concerning their individual products.
2.4.1 What is a Quench
A magnet quench is the breakdown of superconductivity in a partially or fully energized
magnet. The stored field energy is transformed into heat, leading to a fast evaporation of
liquid helium. During a quench, an extremely large quantity of helium gas is produced within a
short time.
Although helium gas is inert, if generated in large enough quantities, it can displace the
oxygen in the room causing potential danger of suffocation (refer to
Emergency Ventilation
During Installation and Quenches [
}
68] ).